The present invention is related to pulse code modulated time division telecommunication systems and, more specifically, to a continuity check tone detector for use with such a system.
A main characteristic of digital telecommunication systems is that subscribers connected to a switching center of the digital telecommunications network in contrast to previous analog telecommunications networks are no longer physically connected with each other when communicating. The generated analog tone signals are sampled, digitalized and arranged in a pulse frame which is processed in the switching center in order to the respective samples be routed to the receiving subscribers. The digital tone samples are converted into analog signals at the receiving end and the original analog signal is reconstructed. This approach requires extensive control information not just for establishing and releasing connections but also for processing of the pulse frames. The control information related to signals on voice transmission paths may be called speech address information and control information regarding procedures, as call connect and release is recognized as signalling formation.
"Common channel signalling" is a further characteristic of various digital telecommunications systems. In this case signalling information is transmitted separate from the speech information and across different transmission paths. The signalling information correspondingly also includes signalling address information in order to specify subunits of the telecommunications system selected to receive corresponding pieces of signalling information.
This structure of having voice transmission paths separated from signalling transmission paths requires individual measures for testing of speech path continuity. Such continuity tests are set up on a link by link basis whereby a connection is set up between a tone generator of the telecommunication system and the continuity check tone detector across a temporary speech transmission path to be tested. A continuous tone is generated, sent across this temporary speech transmission path and received at the far end of the connection by the continuity check tone detector which evaluates tone frequency, level and duration.
In a conventional analog telecommunication system such task would be performed by analog measuring devices. The use of such equipment in conjunction with a digital telecommunication system, however, would unnecessarly complicate this test. The generated analog signal, i.e. the continuous test tone had to be digitalized, transmitted across the selected speech transmission path to be tested, reconverted at the far end and the reconstructed analog signal then had to be measured. Such a design would unnecessarly require additional codecs and filters and affect the stability of the testing system. Filtering of the received analog tone would be necessary in order to eliminate undesired noise and the filtered analog signal had to be thresholded for a root mean square measurement when determining the signal level.